[Zope] ZClass Objectmanager question ...

One more ObjectManager/ZClass question ... When nesting ZClasses, the "SubObjects" tab of the container class says to "Select the kinds of things that can be added to objects of this kind". However, the nested ZClass meta-type does not appear in this selection box. Rather, one proceeds to the "Permissions" tab and changes the class permissions to allow addition of the nested class. I don't understand this. While I know what to do (in recipe fashion), I don't understand the rationale behind it. Cheers, Darran.